On the physics of dust grains in hot gas.

Charging of dust grains in hot (10,000-1 billion K) plasma is studied, including photoelectron and secondary electron emission, field emission, and transmission of electrons and ions through the grain. The resulting grain potentials are (for temperatures of at least about 100,000 K) considerably smaller in magnitude than found by Burke and Silk (1974). Even so, large electrostatic stresses can cause ion field emission and rapid destruction of small grains in very hot gas. Rapid rotation can also disrupt small grains, but damping (by microwave emission) usually limits the centrifugal stress to acceptable values for plasma densities of no more than about 1 per cu cm. Sputtering rates are estimated for grains in hot gas, based upon a semiempirical fit to experimental data. Predicted sputtering rates for possible grain constituents are similar to estimates by Barlow (1978), but in some cases differ significantly. Useful approximation formulas are given for the drag forces acting on a grain with arbitrary Mach number.